The analog-to-digital conversion technique by oversampling which is currently often used is implemented by means of delta-sigma modulators which deliver low-resolution digital words, such as for example 1-bit words, by sampling the analog signals applied at a very high sampling frequency fs (fs=512*F.sub.Nyquist for example), the digital words produced having a mean amplitude which varies in time proportionally to the amplitude of the analog signal applied thereto. A digital filter is connected to the modulator output in order notably to filter the quantization noise, this filter producing digital words with a higher resolution (16 to 24 bits for example) and a much lower frequency, an operation known as decimation. Converters of this type are for example described in U.S. Pat. Nos. 4,866,442, 4,943,807, 4,994,804, etc.
There are notably applications, in the field of acoustics or seismic prospecting, where signals received by sensors have to be combined. Combinations of several signals may be for example achieved after they have been possibly weighted and/or filtered or phase-shifted, or more complex combinations of these multiple signals may be achieved. This is particularly the case in seismic prospecting, where a very great number of receivers is distributed over a zone to be explored in order to receive seismic waves reflected by the subsoil discontinuities in response to jogs caused by the triggering of a source of seismic waves. The receivers most often consist of several elementary sensors interconnected electrically and arranged in the explored zone so as to filter an organized surface noise. Each of these receivers provides a "seismic trace" which is an electrical signal average of the signals produced by the interconnected elementary sensors.
It is well-known that the relative arrangement of the various sensors constituting a single "trace" has a great likelihood of obtaining a good rejection of the surface noise. It is also well-known that, by framing the amplitude and/or the frequency spectrum of the signals produced respectively by the various signals forming a single trace before the combination thereof, it is possible to obtain a considerable attenuation of the noise level and therefore to preserve better the dynamics of the digitization means of the useful signals.
Weighting devices of the active or passive type for changing the amplitudes, frequency spectra or signal phases received by sensors before the combination thereof and which are placed in the vicinity of these sensors are for example described in U.S. Pat. Nos. 2,698,927, 2,747,172, 3,400,783, 3,863,200, 3,863,201.
The main drawback of all the analog preprocessing equipments lies mainly in the lack of flexibility. The filtering parameters must be known in advance and, assuming that the reception device that has been effectively set allows these parameters to be modified, adaptations are also necessary on the site where these local processing means have been installed, which extends the setting operations. Besides, the adjustments which have possibly been achieved before use cannot be changed easily during the recording of the pick up signals.
A technique which is currently used to combine signals consists in converting into digital words of 8, 16 or 32 bits, for example, analog signals to be combined and in utilizing a digital computer programmed to carry out the desired combinations.
This solution may be suitable for a limited number of signals but it becomes highly complex and above all costly for a relatively great number of signals. Within the scope of certain applications, notably in geophysics, where 500 to 4000 different channels are to be taken into account for example, a recording laboratory capable of acquiring all these channels with sufficient dynamics (20 to 24 bits for example) would reach a prohibitive cost.